Cylindrical antenna



S ept 23, 1952 A. ALFORD 2,611,866

CYLINDRICAL ANTENNA Filed June 19, 1946 INVENTOR ANDREW ALFORD Z Wd ATTORNEY t, .v 1'. as; r c. his invention relates to-new and useful im Movements in antenfias and particularly in mobi-letransmitting and receiving antennas foruse automobiles buses trains, aircraft and the likglll LT-"Y D '7 J the' art or radio 'transmission where at least bhttation-is' mobileit is' the general practice to employ vertically polarized radio frequency waves; since they havefgood field strength near to the ground. A simple and wi dely u sed antenna for such waves is a vertical flexible whip antenna. This :atnter rna has several "serious shortcomings. In

oraer ro fui 1ctiorr-erfficie'ntly it mustbe-nearfto; a qiijai'ter'wave *len'g hlong; a length Y at which it is re nant-and Has anarrow pass' -band3 The pass band can bebroadenedby increasing the --thickness of the radiating element but itthen 1 may cease to be a whipantenna and may presen-t'too rigid and' hazardous a projection from a 'mo'ving vehic1e$ The band width f an ordinarv' quarter ave whip antenna" of the order cf- -ten percent ndg even if compensating circuits are'use'di this-can only be increased-to approximately twen- Y-five percenti'( .25-) If the lengthofia whip antenna is'reducedbelow amuarter: wavelength, then :it's ra'diation resistance-will Become material-lylowered and itsinput impedance-will'inclucle capacitive reactance'.- The expedient cfetuning out? this reactance with "a coil :ivill further" :adecrease the efficiency "of the antenna; and it's already small band "width: 'FEfE-Iilti'ng a vertical whipaantenna-away from its perpendicular position; so as toflower the height oft-its tip; willfdimi-nisha both its band width and radiationresistanc'ezi .a-iisAt the relativelyuhigh frequency of 156 m'c'.', a quarter wave-ileng'th 'wh'ip would be all'of 16 inches l n zrrbj 16-inch Iprotrus'ion frcrh the topof 2; vehicle as high as a bus, or a truckfd'r 'a railroad car can be'verry troublesoine; Such vehicles frequently are built nearly as. high-astheminimum headrobmtoleranceilegally permitted for-bridges, tunnels;.rviaducts. 1 and terminals, Moreover; in certain areas such obtrusions would most likely encounter lowwhangin'gkbranches ro'fztrees. s 'Eoldable mounting of a whipantenna to avoid overhead obstacles-willmake*'continued useof-the associated 'radio' equipment impossicIe.-- Fur-thermore; such a.-mountin g ;'a's wellias one'permitting the whip ian'tenna; toi tiltwherfstriking an ohj ect',

will nec'essitatecomplicated and expensive "flex-' quirementswr .r v

A whip antenna attached hicle below -lthe leve1 of Ith'- I running board 'or bumper, :will alwaysbe masked inscme direction by the body of the vehicl r Anbther possible'objection to whipantenn us v that-they are I conspicuous and: easily: identified and may; therefore, constitute an-obvious cme to the-presence of-radio equipmentin afvehicl'e and to thetype oi -vehicle itis,-fo'r xample tha i' aspecial-police car; 41 i w The-protrusion -0fwhip-antennas s 010;] cotton-Z able c -aircraft for the additional reascns that at-ordinary high'=speeds they produce substantial drag; and at very high "speeds theirl drag attains v ordinately I large mag'nitud'es, imam-e vibration will either 'break ofi the antenn or damageth ai-rcraft. F It; l Amongfthe objects of this invbhtidm'arfitdqrb vide a vt'er-tically polarized transmittingor cceiv ing antenna having a bandwidth 'o'f l a'sub stantially circular radiation -pa'tter'n in horiz'on- 133,1 planes andfonziwhichis{ very-compact and therefore will, whe'n.installed on a 'v hcle, -"re'-= sent a-minirnum protrusion." I With this-object in viewl provide a hollo com ducting bodyfsuch as a metalcylinderg w hichi is connectedwith; and" is preferably: carrid by'i a conductingsurface, The-body forms-'a traiismi's'i-i sion: linewith-Fsaid surface whichnray be' the 'oof feeder loeirrg'w conne'ctediwith the' surface and the v bodynear'said' gaps' I: Accordingito notherfeature cf the present invention, when the hollow conducting body s0 proportioned. that the area'ofan' airialcross section of the enclosed space is ofth' brdger of one hal'f ofone-thousandth of a; -wave leneth' squared, then the antenna satisfiesfthe above mentioned objects." .1

1 .Referenceis 3 made to my copendirlg abphcatioii U.'-.S'."Sria1'NoL 669,758; filed M21915, -1946Iiow PatentNoi' 2508,0234; a1thoughparts or i't'" solo 'sure w'il1' be repeated for-the convenience reader and to facilitate the understandingof 'th'e preseht inventicnl 5 Other objects, featuresand advantages git-this invention will become apparent totho'se familiai' .ILM

Fig. 2 is a transverse cross sectional view of the antenna shown in Fig. 1; and

Fig. 3 is a corresponding cross sectional view of a modified form of the antenna shown in Figs. 1 and 2.

Referring now more particularly to Fig. l, the radiating element shown therein consists of a cylindrical body element 10, which may be a metallic casting or spinning shaped like a pie plate. This element hasa topdisc 2, and arim l attached thereto; The cylinder is joined by a tube 3 which is mechanically attached to inside surface of top 2 at substantially its center. In practice tube 3 may be formed as an integral part of element I, 2. Tube 3 may be of metal and as a result will be conductive. However, the inside portion of tube 3 joins areas of the outer-side of disc 2 where B. F. voltages are of zero (or extremely low) potential and, therefore, the inside may be non-conductive without preventing this device from: functioning; On the other hand, it willdozno harmlif .theinside of tube 3 is a'good conductor and, despite 3 that, leakage voltages passingfromthe radiator to tube 3-will be small dueito the-voltage nullon the outer surfacesof this radiator near its center. This is not true of themuter surface. of illLlbB 3'. This surface, which is part'ofqthe inner area of the hollow cylinder should be conductive as there must be alow impedance. axial connection between theinnersurfacesof cylinder 1,12 and a ground plate 4 on whichthe lower; end of tube 3 is mounted. Where a:metallicsupportingmast is used the portionof it-iwhichpasses through the hollow cylinder 1, 2 may take the place of tube 3. Its outer skin will perform this conductive function and there will be:no3need for tube ;3, except that 3 ordinarily servesas a convenient gaugefor properly spacing thecyl nder from.;the Plate 4.

-1;; Tube--3 ;is Lofjsuch length and is so arranged that the cylinder l,'-2 and plate 4 do not touch each otheryit holds them parallel and the rim -of the cylinder evenly separated by gap 6. Gap

fiimayvbezanrairlgap and is sovshownin the drawing; However, it may. be physically sealed by a dielectricsubstance so'long as the area of the gapjstill;.-remains non-conductive, in the ordinary senses-and constitutes a hiatus'in the conductive surfaces of the cylinder wherever it intercepts theme. Gapjfi isshort-circuited by a radial metal platelfi within the cylinder extending from tube 3.-;to rim-l:andcontacting platefi. TThQfiIltll'QzSt IllCtlllG 1-5 may lie-described as a-hollowbody:or cylinder whose ends are closed by disc 2 and plate 4, and whose side wall has a gapyG; which runs around the cylinder in a plane parallel to and-midway between disc 2 and plate land returns onitself.

Theghollow cylinder may be made of solid sheetimetal '(or an equivalent such as solid sheet plastic including-a conductive layer) and, accordingly, the inner and outer areas, will comprise ,wall surfaces physically unbroken except for gapali. However, this physical continuity is not necessary, and the device will operate satisfactorily if made of wire screen or of perforated sheet-.metal if theopenings are not too largewith respect to one Wave length corresponding to the highest operating frequency, ve. g. not larger than 95.00 of a wave length. i 1 ,,The antenna is energized over a-coaxial transmission line having an outer conductor 3 and inner conductor 9. Inner conductor 9 is connectedtoflthe edge of the rim of body element I, and the outer conductor 8 is connected'to plate to the transmission line formed by the edges of I gap 5. Thehollow body portion of this antenna (which may be further described as surrounding and enclosing a toroidal space about the outer perimeter of which gap 6 is located) has distributed inductive -.reactance which is shunted across this transmission line. The magnitude of this distributed reactance depends primarily on the area of a transverse cross section, taken in the plane of bar 5, ofthe toroidalspace onions side of tube :3. This is the space between-the innersurfaces of elements I, 2, theouter surface of tube 3 and thetop surface of plate 4. If the inside diameter of the hollow cylinder is designated' as D, the outer diameter of thetube l3, as d, and the distance between the inner surfaces of top 2 and plate 4 as H, then this cross sectional area may be calculated by using the formula in c ind r wh c lhaveprope ma tudes of distributedreactance and perform in a preferred manneras radiators in accordance-with this-invention have such dimensions that, when converted into wavelengths, thearea derived by solving the equation is A,=.00435w2- Short-circuiting band 5 causes nearly complete reflection of incident waves traveling along gap 5 around both sides of the hollow cylinder toward it. The interference between incident waves and reflected waves results ina standing wave distribution of voltage across gap 6.

Oneminimum ofvoltage across the gap occurs at shorting bar 5; two maxima of voltage occur at points on opposite sides of the hollow cylinder each of which is aboutmidway between bar 5 and the feed point. Under certain conditions, two additional voltage minima, which ordinarily are undesirable, may occur on gap- 6. They will be between bar 5 and the feed .point and are likely to be near to, and on each-side of, the feed point. A condition which may cause this is that the gap is too long (the circumference of the cylinder) in proportion to the cross sectional area A and thattherefore the velocity of propagation along the gap is not great enough for the gap length. I

When the only voltage minima are at the shorting bar and the feed point, the voltage differences across gap 6, though they vary in magnitude, are in the same phase. and currents moving between the edges of gap 6 and the surfaces 2 and 4 move in the same direction.

It has been empirically ascertained in actual tests, and it can be further demonstrated .in other ways, that this in-phase condition ofthesR. F. voltages across gap 6 and the geometric shape of the hollow cylinder will cause this element to act as a horizontally omnidirectional radiator of vertically polarized energy. assuming, of course, that the axis of tube 3 is perpendicular tothe ground.

condenser. n

3&1. 1 #85 .6

w th. this two fe d p in ar n eme t i ta well as by, one in which there is a single feed point-wand a reflectionpoint .or low impedance point diametrically opposite to it. It has been foundthat'usually a double feed affects the operating characteristics of the antenna by broadening the operating frequency band. a

In accordance with the principles described herein the proportions of the antenna affect both the radiation patterns and the input impedance. In certainembodiments where a very close approximation to a circular horizontal radiation pattern is desired, it may be necessary in order to achieve this to use proportions which result in are-active component. This component remains nearly constant over a wide band of frequencies and..therefore, can be avoided by a fixed series .,What I claim is: 1,. ,An antenna in combination with means pro viding a flat conductive surface comprising a hollow conductive body having a peripheral rim spaced closely to the conductive surface forming an air gap between the conductive surface and the peripheral rim in which the material in the peripheral rim, adjacent the air gap forms one side ofa transmission line and the material the conductive surface opposite said rim forms the other side of the transmission line, said air gap having a width not substantially greater than $145k where A is a wave length corresponding to a frequency within the operating band of transmission means within the hollow body supporting :the same from said conductive surface and conductively connecting the hollow body with said-conductive surface and a feeder having one conductor connected to one side of the transmission line and the other conductor connected tothe other side of the transmission line said hollow body having an inside diameter which is'substantially not less than 38k.

2. The antenna according to claim 1-, having a shape in which the transmission line forms a closed loop.

' 3. The antenna according to claim 1, in which the hollow conducting body has a circular perimeter whereby the transmission'line forms a circular loop.

4.-='Ihe antenna'according'to claim 1, in which the surface-and edge of the rim are in different planes closely spaced one from the other.

5.' 'The antenna according to claim 1, having ashape in which the transmission line forms a closed loop, and having a short circuit means across the line at a point half the way around the loop from where the terminals of said feeder are connected.

' :6. The antenna according to claim 1, and in which the rim forming apart of the transmis-' sion line has the shape of a short cylinder, and short circuit means across the line substantially diametrically opposite the point of connection of the feeder.

-7. An antenna comprising a conducting cylinder open at one end, a conducting surface, means providing a conductor extending axially within the cylinder and having one end fastened to said surface and holding the cylinder with the edge of the open end thereof in spaced relationship to said conductive surface to form a gap between them; the cross sectional area of the space within the cylinder taken in a plane containing its a ds and on one side of the conductor being substantially one-half-of one-hundredth of a wave len th SQ IB fiQ P I lE o t e en er f 8. quency f th o er tin band. a de avin two conductors one conductor connected with the surface and the other conductor with the cylinder near said gap, and short circuit means across the gap at a point diametrically opposite thepoints of connection of the transmission line.

.3. An antenna comprising a metal cylinder open at one and closed at the other end, said cylinder having a circular cross section, a plate providing a metal surface, a metal tube extending axially within the cylinder and having one end fastened to the closed end thereof and the other end to the surface holding the latter. in spaced relationship with the edge of the open cylinder end to form a circular gap between them, the cross sectional area of the toroidal space within the cylinder taken in a plane containing the axisof said tube and on one side thereof being substantially one-half of onehundredth ora wave length squared corresponding to the center frequency of the operating band, a feeder having two conductors with one conductor connected with the plate and the other conductor with the'cylinder near said gap, and short circuit means across the gap at a point diametrically opposite the points of connection of the feeder. H

9. An antenna comprising a metal cylinder open at one and closedat the other end, said cylinder having a circular cross section, a metal plate, a metal tube having one end fastened to the closed end of the cylinder and the other end to the plate and holding the edge of the open cylinder end above the plate to form a gap between the edge and the plate, a transmission line having one conductor connected with the plate and the other conductor with the cylinder near saidgap, and short circuit means across the gap at a point diametrically opposite the points of connection of the transmission line.

10. The antenna accordingto claim 9, and in which the cross sectional area of the toroidal space within the cylinder taken in a plane containing the axis of said tube and on one side thereof is substantially .00ei3k where A is the wave length corresponding to the center frequency of the operating band.'

11. An'antenna comprising a metal cylinder open at one .andclosed at the other end, said cylinder having a circular cross section of a diameter of about one-half a wave length corresponding'to' the center frequency of the operating band, a metal plate, a metal tube having one end fastened to the closediend of the cylinder and the other end to the plate holding the edge of the open cylinder endabove the plate, the circulargap between the edge and the plate being a small fraction of a 'wave length wide, a transmission line having one conductor connected with the plate and the other conductor with the cylinder at the gap, and short circuit means across the gap at a point diametrically opposite the points of connection of the transmission line.

l2. An antenna comprising a metal cylinder 05x high where A is the wave length corresponding to the center frequency of the operating band, said cylinderv being open at one and closed at the otherend, said cylinder having diameter substantially .447\, a. metal plate, a metal tube having a diameter substantially .27A and having one end fastened to the closed end of the cylinder and the other end to the plate thereby holding the edge of the open cylinder end above the plate at a distance to form a gap substantially ..0 45r wide between the edge of the cylinder and the plate, a concentric transmission line having the outer conductor connected with the plate and the inner conductor with the cylinder near said gap, and a short circuit means across the gap at a point diametrically opposite the points of connection of the transmission line.

13. An antenna comprising a metal cylinder, a metal plate closing and supporting one end of the cylinder, a metal tube axially positioned within the cylinder having one end fastened to the plate, a disc having substantially the same area as a transverse cross section of the cylinder carried by the other end of the tube and held by it at a distance from the edge of the open cylinder end parallel with the plate to form a circular gap, a transmission line having one conductor connected with the disc and the other conductor with the cylinder near said gap, and a short circuit means across the gap at a point diametrically opposite the points of connection of the transmission line.

14. The antenna according to claim 13, and in which the cross sectional area of the space within the cylinder taken in a plane containing the axis of said tube and on one side thereof is substantially one-half of one-hundredth of a wave length squared, the wave length corresponding to the center frequency of the operating band.

15. The antenna according to claim 13, and in which the plate is larger in area than the transverse cross sectional area enclosed within the perimeter of the cylinder.

16. The antenna according to claim 13, and in which the cross sectional area of the toroidal space within the cylinder taken in a plane containing the axis of said tube and on one side thereof is substantially one-half of one-hundredth of a wave length squared which corresponds to a frequency within the operating band, and the transmission line projects through a hole in the plate adjacent the side wall of the cylinder.

17. An antenna comprising a metal cylinder open at one and closed at the other end, said cylinder having a circular cross section, a metal disc, a metal tube having one end fastened to the closed end of the cylinder and the other end 'to the disc and holding the disc above the edge of the open cylinder end but spaced therefrom to form a circular gap between the edge and the disc, a transmission line having one conductor connected with the disc and the other conductor with the cylinder near said gap, and a short circuit means across the gap at a point diametrically opposite the points of connection of the transmission line.

18. In an antenna, a hollow cylinder having closed ends and a gap formed in the cylindrical Wall at one end in the form of a loop, connection means between the ends near the axis of the cylinder, means for feeding the cylinder near the gap, and short circuit means across the gap, the distances from said short circuit means along the gap to said feed point being less than the normal distance from the short circuit to the point where a voltage minimum would occur.

19. In an antenna, a hollow cylinder having closed ends and a gap formed in the side wall at one end in the form of a loop, connection means between the ends near the axis of the cylinder, means for feeding the cylinder near the gap, and short circuit means across the gap at substantially the same distance from the feed point measured in opposite directions along the gap, said distance being less than that necessary to establish a voltage minimum along the gap. 20. The antenna according to claim 1, having a low impedance radio frequency reflecting means across the transmission line at its mid-point from the connection of the feeder across the transmission line.

ANDREW ALFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,078,234 Buschbeck Apr. 27, 1937 2,235,506 Schelkunoff Mar. 18, 1941 2,284,434 Lindenblad May 26, 1942 2,297,513 Von Baeyer Sept. 29, 1942 2,423,150 Lindenblad July 1, 1947 2,463,547 Meier Mar. 8, 1949 

